Method of crystallizing



Sept. 27, 1960 w. c. BAUER ET AL METHOD OF CRYSTALLIZING Filed Dec. 13, 1954 INVENTORS. WILLIAM C. BAUER ALLEN P N1 CUE y KENNETH c. RULE g%a,)- ryz,4/) AGENT m d mm United States Patent METHOD OF CRYSTALLIZING William C. Bauer and Allen P. McCue, Green River, Wyo., and Kenneth C. Rule, Noroton Heights, Comp, assignors to Food Machinery and Chemical Corporation, New York, N.Y., a corporation of Delaware Filed Dec. 13, 1954, Ser. No. 474,828

8 Claims. ((1 23 -300) This invention relates 'to improvements in the method of crystallizing certain inorganic salts, particularly those containing sodium and bicarbonate ions, and more particularly sodium sesquicarbonate.

As is well known, when the solubility of an inorganic salt in a solvent is exceeded, as by sufliciently lowering the temperature of an unsaturated solution or by introducing an excess of one or more of the ions involved,'the salt is deposited in solid form. This method is perhaps the most commonly used process for the production of crystalline salts, and is exemplified in the production of sodium sesquicarbonate, where impure trona mineral is mined and purified by recrystallization from a hot, aqueous solution as described, for example, in the Pike US. Patent No. 2,639,217.

In many cases, the type of salt crystals obtained by v such methods are of decidedly inferior quality, considering such factors as crystal shape, purity, settling rate, size, uniformity, dewaterability, resistance to breakage and bulk density. The attainment of the desired'results as to these characteristics has long been a problem in the production of sesquicarbonate from trona, which product is largely converted to soda ash as the final market commodity. Inferiority of the intermediate sesquicarbonate ordinarily leads to inferiority in the final soda ash product.

It is accordingly an object of the present invention to provide a novel method for obtaining sodium sesquicarbonate of greatly improved quality.

Another object of the present invention is to provide such a method which is practical and economically feasible.

Other objects and advantages of the. invention will appear from a consideration of the following disclosure and the attached illustrations.

We have discovered that the shortcomings and disadvantages of the prior art methods of crystallizing sodiumsesquicarbonate are largely eliminated by the use of very small concentrations of an anionic-active surface active agent. Adopting the name commonly used by chemists, surface active agents are herein called surfactants.

The preferred types of these anionic-active surfactants are organic sulfate (organosulfate) or sulfonate (organosulfonate) derivatives, and of these preferred subclasses, the preferred sulfonates are alkyl benzene or alkyl naphthalene sulfonates wherein alkyl carbons total at least four and desirably more, and the preferred sulfates are the higher alkyl alcohol sulfates. Thus, particularly effective compounds are dodecyl benzene sulfonate and polypropylene benzene sulfonates ranging from C1048 alkyl groups; and dibutyl or diisopropyl naphthalen sulfonate.

Another preferred sub-class, related to the foregoing preferred sub-classes, are the taurates derived from N- alkyltaurine (RNHCH CH SO H) and fatty acids, containing a fatty acid residue of substantial length, i.e., at least 8 carbon atoms. Examples are sodium-N-methyl-N- lauryl taurate, sodium-N-cyclohexyl-N-palmityl taurate, sodium-N-methyl taurate of tallow acids and sodium-N- methyl-N-oleyl taurate. Taurates containing lower ice molecular weight fatty acid substituent groups are proportionately less effective.

The primary alcohol sulfates containing alkyl groups of substantial size, such as those based on lauryl alcohol, are very effective additives. Examples are sodium, ammonium and triethanolamine lauryl sulfates. Primary alcohol sulfates containing smaller alkyl groups, e.g. on the order of only C alkyl groups, are proportionately less effective and are not recommended. The practical upper limit is about C groups.- Examples in the C range include: sodium octyl sulfate, sodium lauryl sulfate, ammonium lauryl sulfate, triethanolamine lauryl sulfate, sodium coconut alcohol sulfate, sodium tridecyl alcohol sulfate, sodium tallow alcohol sulfate, sodium cetyl sulfate and sodium oleyl sulfate.

It appears that in general the free acids of these various surfactants may be used, because they are converted to the sodium salts in the process liquors, which are mildly unique efiectiveness of the anionic-active surfactantsparticularly the subclasses above enumeratedand the complete lack of effectiveness of the cationic and nonionic classes. In general these theories have all failed to fully explain this unexpected result.

Up to about parts per million (p.p.m.) of surfactant are recommended in the crystallization of sodium sesquicarbonate, but for various reasons the preferred concentration is about 5-20 ppm. Levels below 5 ppm. show beneficial efiect, but are not the preferred concentration. Similarly, amounts in excess of 100 p.p.m. may be used with beneficial results, but complicating effects may begin to appear at excessively high levels, such as crystal twinning or branching, and contamination of the product, and the use of these higher concentrations is, of course, economically undesirable.

The levels specified herein refer to the active content of the various commercial preparations available, unless otherwise noted.

In the process of refining trona to produce pure sodium sesquicarbonate as described, for example, in said Pike US. Patent No. 2,639,217, the surfactant is preferably in troduced just ahead of the cooling and crystallization step, so that less is lost in any mother liquor discarded from the recycling system, or removed by any adsorbent used in the purification procedure.

Prior to the instant discovery, the trona process of making sodium sesquicarbonate as described, for example, in said Pike US. Patent No. 2,639,2l7 in a commercial plant was operating at only about 25-30% of the designed capacity, due to trouble in the operation of the centrifuges which separate the sodium sesquicarbonate crystals from the mother liquor.

This difiiculty arose from the fact that the crystalline slurry obtained in the cooling and crystallization step had very undesirable properties. Thus, it is desirable that this slurry be rich in crystal content, and yet the crystal content cannot be so high thatthe slurry fails to flow properly. Also, the crystals must be of such a nature that they can be readily separated from the slurry in the centrifuges, i.e. they must settle readily and be easily dewaterable. However, none of these desirable characteristics was being obtained, and as a consequence the centrifuges were the bottlenecks in the plant operations.

Considerable effort was expended in changing operating methods which, it was believed, might have some efiect upon the characteristics of the crystalline slurry or the crystals themselves, including various efiorts at im- The crystals continued to be very smalhandfneedle shaped, and were agglomerated into clumps-.of'feathery appearance. The three points in the process where their undesirable structure was especially troublesome were. in.

the settlers, in the concentrating feeders, and in thecenr trifuges. Due to poor settling the net result was that a, low concentration slurry was being fed into. the centrifuges, which in turn delivered a poorly dewatered. product at far below designed capacity rates.

Thus, typical slurries contained on the order, of'as little as 35-40 percent crystals by weight, and flowed.

very. sluggishly and unevenly. The use of about 25 ppm. of'one of'the preferred additives gave slurries containing 50-5.S percent crystals by weight 'which, even though containing much more solid material, pumped. and flowed.

smoothly and evenly.

Other undesirable consequences due to the existence of this problem were fragility of the crystals, leading to excessive. breakage in centrifuging; nonuniformity in particle size; excessive dustiness of the soda ash product due to fine crystals; low bulk density due to fine, feathery crystals; and an unsatisfactory degree of purity due. to in: complete removal of mother liquor in the centrifuge. operation. a

The use of additives in the trona purification process. as" described, for example, in said Pike US. Patent No. 2,639,217, did not initially appear attractive, since it, was expected that high concentrations of additive in, the process' liquors would be necessary at the moment of crystal: lization; in order to give any substantial benefit. Difficulty in this respect was anticipated because the process. liquor. ispurified'with an adsorbent and filtration justaheadof. the crystallization step, and it was realized that this, ad..- sorbent' would probably effectively remove any residual additivecarried along in the recycled mother liquor. This. meant thatthe additive would have to be introduced anew inall of-thepurified liquor going to the crystallizer, which, WQ1l1d".ObVlOl-1S1Y entail a substantial additional processing cost;

A review. of the published work done by others indicated that a wide range of additives had been tried, in.

various crystallization problems, but no pattern existed which would allow reliable predictions as to the type. of, additive which would be effective. Thus, ionic additives such as sodium nitrate and sodium chloride had been tested for their effect on the crystallization of sodium. bicarbonate, while at the other extreme the effect of nonionie additives had been noted in the crystallization of other substances. As is well known, there is, no, generally accepted theory on the mechanism of crystallization and, as had been noted by one. of the well-known chemical: encyclopedias, the entire field of crystallization is, highly empirical.

Numerous mechanisms have been suggested. for the. eifects on aqueous solutions produced by the. addition of a surface active agent. One theory isv that areductionin surface tension around the crystal, i.e., atthe solid-liquid, interface, allows, easier and more rapid deposition, creatinga better balance between crystal growth and nucleation. Another theory is that the additive interferes with or inhibits growth at the ends of thecrystal, with the resulting tendency to produce shorter, thicker crystals in,-. stead of. long, slender crystals which would subsequently breakup tofurnish additional nuclei. Other workershave theorized that surface tension is not a substantial factor or that, if it is, it is operative-only on, crystals forming at" the air-liquid interface.

The invention will be better understood from a consideration ofzthev following examples, and a reference to theiattachediillustrations. These illustrations: are reflection- 4; photomicrographs of various crystalline materials which clearly demonstrate the invention describedand claimed herein, and are identified as follows:

Sodium sesquicarbonate crystals taken at 10 power 10X magnification.

Figure 1 shows crystals of sesquicarbonate prepared in the laboratory Without the use. of. a surfactant.

Figure 2 shows the, rod-like sesquicarbonate. crystals,

preparedwith the useof 150 ppm. offdodecyh benzene sodium sulfonate.

Figure 7 3 shows sesqnicarbonate crystals prepared :on. a plant scale without the use of additives.

Figure 4 shows the rod-like sodium sesquicarbonate crystals prepared ona plant scale with the use of 10 p.p.m. of the same surfactant as in Figure 2.

Example 1 out, the use of, an additive, the settled slurry contained about 121-20 percent crystals. by weight andfhad, a. de- Waterability,.measured by the amountof, water retain d inthecentrifuge cake,-of about 8-13 percent; These crystals are: shown in Figure 1.

Figure 2 The procedure described in Example '1' was repeated, butthis timean additive was introduced, into the solution prior to cooling. The additive used'was dodecyl'benzene sodium. sulfonate, and enough of thev commercial material (Detergent D.40 or Sulfonate AA-9) Wasused to give a concentration of about 150. ppm. The de waterability was improved to a value of about; 3-4 percent-- and the settled slurry had a crystal content of about 40 percent. When this run was repeated at an additive. concentration of 80 p.p.m., the dewaterability was about;6 percent and the settled slurry contained about 35 percent crystals by weight. See Figure 2.for illustration, andgnote the striking improvement in comparison'with'Figure 1..

Example j Another run similar to the foregoing example, but'using butyl naphthalene sodium sulfonate (Sorbit AC, containing about percent active material) likewise gave greatly improved results over Example 1,.g1ving a dewaterability of about 41percent and a settledsluriycontaining about 30 percent crystals by weight, when 330* p.p.m. ofthe commercial additive was used;

Example 4 Another run, similar to the foregoing, wasmade using 400 ppm. of commercial sodium lauryl sulfateand thistime a dewaterability of 3.6 percent and'asettled slurryof 39 percent crystal content was obtained.

Example 5 The laboratory use ofabout 380 ppm. of commercial triethanolamine lauryl sulfate gave a dewaterability of 3.1 percent and a 35 percent crystal content settled slurry.

These crystallizations were efiectedin a 5.0.0

It was found desirable.

ability of 3.6 percent and a settled slurry of 39 percent crystal content.

Example 7 The use of 140 p.p.m. of sodium-N-methyl-N-oleyltaurate (Igepon T-77, containing 72 percent active material) gave a dewaterability of about 4 percent and a settled slurry of about 37 percent crystal content, in small scale trials.

Example 8 The troubles encountered in the operation of the plant process for the production of crystalline sesquicarbonate have been described above. Without the use of an additive, the plant centrifuge cake had a moisture content of about -15 percent, with the centrifuge turning out about 210 tons per day (t.p.d.) of sesquicarbonate. This centrifuge production Was only about one-third of the designed capacity, and the product had a higher moisture content than contemplated in the design calculations. See Figure 3 for crystal form.

Example 9 The plant operation was modified by the introduction of dodecyl benzene sodium sulfonate just ahead of the cooling and crystallization step, at a concentration in the plant liquors of about 80 p.p.m. (active material). After operations had reached equilibrium conditions, the centrifuge cake moisture dropped to about 7 percent, with the centrifuge production increased to about 420 t.p.d. Further runs showed that production could be raised to 850 t.p.d., substantially exceeding the designed capacity of 650 t.p.d. of sesquicarbonate.

Example 10 Operation of the plant in a manner similar to that described in Example 9, but at an additive level of only 20 p.p.m. (active material), resulted in centrifuge cake moistures of about 6 percent, and a centrifuge production rate of about 500 t.p.d. Further runs showed that even at this additive level and moisture content a production rate of 850 t.p.d. can be maintained.

Example 11 A run similar to Example 10 was made at a level of 10 p.p.m. surfactant, giving excellent rodlike crystals, which, as shown in Figure 4, have a length several times the greatest Width.

Example 12 Operation of the plant in a manner similar to that described in the two preceding examples, but at an additive level of only about 5 p.p.m. (active material), gave centrifuge cake moistures of about 9 percent and centrifuge capacities of about 850 t.p.d.

Example 13 Using the surfactant of Example 6 in the plant at an active material concentration of 14 p.p.m. gave results very similar to those obtained in Example 11.

In the case of sesquicarbonate, it was found that increasing the scale of operations permitted the use of considerably lower concentrations of additive, without substantially affecting the numerous advantages realized from the practice of this invention, and in most cases actually giving superior results.

It should be understood that the foregoing descriptive and illustrative materials represent only typical embodiments of the invention, and are not to be construed as limiting the scope thereof, as defined by the claims which follow.

That which is claimed as patentably novel is:

1. A process for preparing crystals of sodium sesquicarbonate which crystals are improved in size, dewatering ability and settling rate, which comprises forming a plant solution of sodium sesquicarbonate by dissolving crude trona in a recycling mother liquor and crystallizing sodium sesquicarbonate from said solution by effecting the crystallization of said salt from said solution in the presence of about 5 to about 100 parts per million of an anionic active surfactant selected from the group consist- 5 ing of (1) alkyl benzene sulfonates containing at least 8 alkyl carbon atoms, (2) alkyl naphthalene sulfonates containing at least 4 alkyl carbon atoms, (3) primary alkyl alcohol sulfates containing at least 10 carbon atoms, and (4) N-substituted taurines of the formula where R is a hydrocarbon radical, R" is the acyl radical of a higher fatty acid and M is an alkali metal.

2.- The process of claim 1 wherein the salt being crys- 15 tallized is sodium sesquicarbonate and the surfactant is present in the aqueous solution prior to crystallization in a concentration of from about 5 to about parts per million active material.

3. The process of claim 1 wherein the surfactant is an alkyl benzene sulfonate containing at least 8 carbon atoms.

4. The process of claim 1 wherein the surfactant is an alkyl naphthalene sulfonate containing at least 4 alkyl carbon atoms.

5. The process of claim 1 wherein the surfactant is a primary alcohol sulfate containing at least 10 carbon atoms.

6. The process of claim 1 wherein the surfactant is an N-substituted taurine of the formula where R is a hydrocarbon radical, R" is the acyl radical of a higher fatty acid and M is an alkali metal.

7. A process of preparing large, uniform-sized, easily 5 dewatered crystals of sodium sesquicarbonate by initial crystallization from a saturated aqueous plant solution of trona in the presence of about 5 to about 80 parts per million active material of a branched alkyl chain dodecyl benzene sulfonate and separating said crystals from said saturated solution.

8. A process for preparing substantially pure sodium sesquicarbonate composed of large individual, substantially non-adherent, long rod-like crystals which are substantially free of fines, said sodium sesquicarbonate being derived from crude trona by crystallization from a purified plant liquor burdened with dissolved trona and having sodium sesquicarbonate as the stable crystal phase in the presence of about 5 to about 100 parts per million of a surface active crystal promoting agent, selected from the group consisting of (1) alkyl benzene sulfonates containing at least 8 alkyl carbon atoms, (2) alkyl naphthalene sulfonates containing at least 4 alkyl carbon.

atoms, (3) primary alkyl alcohol sulfates containing at least 10 carbon atoms and (4) N-substituted taurines of the formula R'R"NCH CH SO M whereR is a hydrocarbon radical, R" is the acyl radical of a higher fatty acid and M is an alkali metal, said crystals having a length several times the greatest width and having an average over-all size at least several times that of crystals crystallized from a similar liquor in the absence of said agent.

References Cited in the file of this patent UNITED STATES PATENTS 1,766,705 Dehnel June 24, 1930 2,346,140 Pike Apr. 11, 1944 2,591,704 King Apr. 8, 1952 2,595,238 Frejacques May 6, 1952 2,607,660 Robertson Aug. 19, 1952 2,670,269 Rahn Feb. 23, 1954 2,704,239 Pike Mar. 15, 1955 2,720,446 Whetstone Oct. 11, 1955 OTHER REFERENCES I. W. Perry et al.: Surface Active Agents, published by Interscience Publishers, Inc., New York, 1949, p. 102. 

1. A PROCESS FOR PREPARING CRYSTALS OF SODIUM SESQUICARBONATE WHICH CRYSTALS ARE IMPROVED IN SIZE, DEWATERING ABILITY AND SETTLING RATE, WHICH COMPRISES FORMING A PLANT SOLUTION OF SODIUM SESQUICARBONATE BY DISSOLVING CRUDE TRONA IN A RECYCLING MOTHER LIQUOR AND CRYSTALLIZING SODIUM SESQUICARBONATE FROM SAID SOLUTION BY EFFECTING THE CRYSTALLIZATION OF SAID SALT FROM SAID SOLUTION IN THE PRESENCE OF ABOUT 5 TO ABOUT 100 PARTS PER MILLION OF AN ANIONIC ACTIVE SURFACTANT SELECTED FROM THE GROUP CONSISTING OF (1) ALKYL BENZENE SULFONATES CONTAINING AT LEAST 8 ALKYL CARBON ATOMS, (2) ALKYL NAPHTHALENE SULFONATES CONTAINING AT LEAST 4 ALKYL CARBON ATOMS, (3) PRIMARY ALKYL ALCOHOL SULFATES CONTAINING AT LEAST 10 CARBON ATOMS, AND (4) N-SUBSTITUTED TAURINES OF THE FORMULA 